Originally Posted by Soldier
At the last second you try to straighten out your back. Unfortunately some bad stuff is happening when you do that. When you straighten out it pushes your upper body forwards over the bar which puts your shoulders out of the bar. That tends to force people to get out on their toes and shifts all the weight onto the lower back.
When you straighten out your back, do it by bending your knees more and pulling your butt towards the ground slightly. Same idea, different implementation. Another way to think about it is to straighten your back by pushing your stomach closer to the top of your thighs.
Don't worry about arching your back. Your spine is more or less neutral and that's fine.
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